The economists–Alan Benson of the University of Minnesota, consultant Raimundo Krishna Esteva, and Frank Levy of MIT–closely examined tuition, borrowing, and administrative data from the more selective University of California system and the less selective California State University (CSU) system, along with census data.

Their calculations, for instance, factor in the substantial risk that an entering student will drop out or that he or she will take longer than four years (and thus pay more in tuition) to earn a degree, as well as the painful reality that tuition costs are rising while wages, even for college graduates, haven’t been. In the U-C system, the latest data show that 20% of students fail to get a bachelor’s of arts within six years. At California State, it’s 52%.

While college is still worth the money on average, they say that “more graduates now run the risk of not earning enough to make their investment in college worthwhile.” By their new estimates, a four-year degree was still a good investment for the typical student at the University of California and for women at California State, but for the typical male CSU student, they call a bachelor’s degree only “a marginally good investment.”

And the investment overall is increasingly risky. In 2000 and earlier, there was a “miniscule possibility” that a freshman in either system would eventually have difficulty repaying a loan from his or her wages. By 2010, rising tuition and a deteriorating job market pushed the probability of repayment difficulties for students at the costlier University of California system to 7% for men and 15% for women, “large enough to pose a significant risk for families with limited resources.” For students at the cheaper California State system, debt risks remained minimal.

College, the economists conclude, is “a stepping stone, not a ticket, to the middle class.” Like any investment, it deserves scrutiny.